Prediction of particle size distribution in suspension polymerization reactors: effect of turbulence nonhomogeneity

The quantitative description of particle size distribution development in s uspension polymerization reactors is very complex. The exact mechanisms of breakage and coalescence/aggregation of the polymerizing drops are generall y not very well understood, and are closely related and controlled by the s pectrum of turbulent energy dissipation rate in the reactor. In the present investigation, a two-compartment population balance model was developed fo r taking into account the large spatial variations of the local turbulent k inetic energy, in order to predict the evolution of droplet sizes in a high holdup (i.e., 47-50 vol%) suspension polymerization system as a function o f the most important process conditions, such as type of suspending agent, monomer/water-phase ratio, polymerization temperature, quality of agitation , and evolution of the dispersed-phase density, interfacial tension and vis coelasticity during the polymerization. Phenomenological expressions of the literature were modified for drops in the viscous dissipation range and we re applied for describing the breakage and coalescence rates of the polymer izing dispersed phase as a function of the basic hydrodynamics and evolving physical properties of the system. Computational fluid dynamics simulation s were used for estimating the volume ratio of the impeller and circulation regions, the ratio of turbulent dissipation rates and the exchange flow ra te of the two compartments at different agitation rates and continuous-phas e viscosities. The theoretical model can predict reasonably well the experi mentally observed inhomogeneities of the drop size distribution as well as the evolution of particle size distribution in VCM suspension polymerizatio n, especially considering the various assumptions in formulating the drop b reakage and coalescence rates and in the two-compartment approximation of t he inhomogeneities of the turbulent flow field in the suspension polymeriza tion reactor. (C) 2000 Elsevier Science Ltd. All rights reserved.